1. Field of the Invention
This invention relates to methods and apparatus for generating a signal having at least one desired output frequency utilizing a bank of vibrating micromechanical devices.
2. Background Art
The need for passive off-hip components has long been a key barrier against communication transceiver miniaturization. In particular, the majority of the high-Q bandpass filters commonly used in the RF and IF stages of heterodyning transceivers are realized using off-chip, mechanically-resonant components, such as crystal and ceramic filters and SAW devices, as illustrated in FIG. 1. Due to higher quality factor Q, such technologies greatly outperform comparable filters implemented using transistor technologies, in insertion loss, percent bandwidth, and achievable rejection. High Q is further required to implement local oscillators or synchronizing clocks in transceivers, both of which must satisfy strict phase noise specifications. Again, as illustrated in FIG. 1, off-chip elements (e.g., quartz crystals) are utilized for this purpose.
Being off-chip components, the above mechanical devices must interface with integrated electronics at the board level, and this constitutes an important bottleneck against the miniaturization of super-heterodyne transceivers. For this reason, recent attempts to achieve single-chip transceivers for paging and cellular communications have utilized alternative architectures that attempt to eliminate the need for off-chip high-Q components via higher levels of transistor integration. Unfortunately, without adequate front-end selectivity, such approaches have suffered somewhat in overall performance, to the point where they so far are usable only in less demanding applications.
Given this, and recognizing that future communication needs will most likely require higher levels of performance, single-chip transceiver solutions that retain high-Q components and that preserver super-heterodyne-like architectures are desirable.
Recent demonstrations of vibrating beam micromechanical (xe2x80x9cxcexcmechanicalxe2x80x9d) resonator devices with frequencies in the VHF range and Q""s in the tens of thousands have sparked a resurgence of research interest in communication architectures using high-Q passive devices as disclosed in the above-noted patent application entitled xe2x80x9cDevice Including A Micromechanical Resonator Having An Operating Frequency and Method of Extending Same.xe2x80x9d Much of the interest in these devices derives from their use of IC-compatible microelectromechanical systems (MEMS) fabrication technologies to greatly facilitate the on-chip integration of ultra-high-Q passive tanks together with active transistor electronics, allowing substantial size reduction.
FIG. 2 illustrates a comparison of MEMS and SAW wherein MEMS offers the same or better high-Q frequency selectivity with orders of magnitude smaller size. Indeed, reductions in size and board-level packaging complexity, as well as the desire for the high performance attainable by super-heterodyne architectures, are principal drivers for this technology.
Although size reduction is certainly an advantage of this technology (commonly dubbed xe2x80x9cRF MEMSxe2x80x9d), it merely touches upon a much greater potential to influence general methods for signal processing. In particular, since they can now be integrated (perhaps on a massive scale) using MEMS technology, vibrating xcexcmechanical resonators (or xcexcmechanical links) can now be thought of as tiny circuit elements, much like resistors or transistors, in a new mechanical circuit technology. Like a single transistor, a single mechanical link does not possess adequate processing power for most applications. However, again like transistors, when combined into larger (potentially, VLSI) circuits, the true power of xcexcmechanical links can be unleashed, and signal processing functions with attributes previously inaccessible to transistor circuits may become feasible.
The Need for High Q in Oscillators
For any communications application, the stability of the oscillator signals used for frequency translation, synchronization, or sampling, is of utmost importance. Oscillator frequencies must be stable against variations in temperature against aging, and against any phenomena, such as noise or microphonics, that cause instantaneous fluctuations in phase and frequency. The single most important parameter that dictates oscillator stability is the Q of the frequency-setting tank (or of the effective tank for the case of ring oscillators). For a given application, and assuming a finite power budget, adequate long- and short-term stability of the oscillation frequency is insured only when the tank Q exceeds a certain threshold value.
Given the need for low power in portable units, and given that the synthesizer (containing the reference and VCO oscillators) is often a dominant contributor to total transceiver power consumption, modern transceivers could benefit greatly from technologies that yield high-Q tank components.
The Need for High Q in Filters
Tank Q also greatly influences the ability to implement extremely selective IF and RF filters with small percent bandwidth, small shape factor, and low insertion loss. As tank Q decreases, insertion loss increases very quickly, too much even for IF filters, and quite unacceptable for RF filters. As with oscillators, high-Q tanks are required for RF and IF filters alike, although more so for the latter, since channel selection is done predominantly at the IF in super-heterodyne receivers. In general, the more selective the filter, the higher the resonator Q required to achieve a given level of insertion loss.
Micromechanical Circuits
Although mechanical circuits, such as quartz crystal resonators and SAW filters, provide essential functions in the majority of transceiver designs, their numbers are generally suppressed due to their large size and finite cost. Unfortunately, when minimizing the use of high-Q components, designers often trade power for selectivity (i.e., Q), and hence, sacrifice transceiver performance. As a simple illustration, if the high-Q IF filter in the receive path of a communication subsystem is removed, the dynamic range requirement on the subsequent IF amplifier, IQ mixer, and A/D converter circuits, increases dramatically, forcing a corresponding increase in power consumption. Similar trade-offs exist at RF, where the larger the number or greater the complexity of high-Q components used, the smaller the power consumption in surrounding transistor circuits.
The Micromechanical Beam Element
To date, the majority of xcexcmechanical circuits most useful for communication applications in the VHF range have been realized using xcexcmechanical flexural-mode beam elements, such as shown in FIG. 2 with clamped-clamped boundary conditions. Although several micromachining technologies are available to realize such an element in a variety of different materials, surface micromachining has been the preferred method for xcexcmechanical communication circuits, mainly due to its flexibility in providing a variety of beam end conditions and electrode locations, and its ability to realize very complex geometries with multiple levels of suspension.
U.S. Pat. No. 6,049,702 to Tham et al. discloses an integrated passive transceiver section wherein microelectromechanical (MEM) device fabrication techniques are used to provide low loss, high performance switches. Utilizing the MEM devices also makes possible the fabrication and use of several circuits comprising passive components, thereby enhancing the performance characteristics of the transceiver.
U.S. Pat. No. 5,872,489 to Chang et al. discloses an integrated tunable inductance network and method. The network utilizes a plurality of MEM switches which selectively interconnect inductance devices thereby providing a selective inductance for a particular circuit.
U.S. Pat. No. 5,963,857 to Greywall discloses an article comprising a micromachined filter. In use, the micromachined filters are assembled as part of a radio to miniaturize the size of the radio.
U.S. Pat. Nos. 5,976,994 and 6,169,321 to Nguyen et al. disclose a batch-compatible, post-fabrication annealing method and system to trim the resonance frequency and enhance the quality factor of micromechanical structures.
U.S. Pat. Nos. 5,455,547; 5,589,082 and 5,537,083 to Lin et al. disclose microelectromechanical signal processors. The signal processors include many individual microelectromechanical resonators which enable the processor to function as a multi-channel signal processor or a spectrum analyzer.
U.S. Pat. No. 5,640,133 to MacDonald et al. discloses a capacitance-based, tunable, micromechanical resonator. The resonators may be selectively tuned and used in mechanical oscillators, accelerometers, electromechanical filters and other electronic devices.
U.S. Pat. No. 5,578,976 to Yao, U.S. Pat. No. 5,619,061 to Goldsmith et al. and U.S. Pat. No. 6,016,092 to Qiu et al. disclose various micromechanical and microelectromechanical switches used in communication apparatus.
U.S. Pat. No. 5,839,062 to Nguyen et al. disclose a MEMS-based receiver including parallel banks of microelectromechanical filters.
U.S. Pat. Nos. 5,491,604 and 5,955,932 to Nguyen et al. disclose Q-controlled microresonators and tunable filters using the resonators.
U.S. Pat. No. 5,783,973 discloses a micromechanical, thermally insensitive silicon resonator and oscillator.
The following articles are of general interest: Nguyen et al., xe2x80x9cDesign and Performance of CMOS Micromechanical Resonator Oscillatorsxe2x80x9d, 1994 IEEE International Frequency Control Symposium, pp. 127-134; Wang et al, xe2x80x9cQ-Enhancement of Microelectromechanical Filters Via Low-Velocity Spring Couplingxe2x80x9d, 1997 IEEE Ultrasonics Symposium, pp. 323-327; Bannon, III et al., xe2x80x9cHigh Frequency Microelectromechanical IF Filtersxe2x80x9d, 1996 IEEE Electronics Devices Meeting, San Francisco, Calif., Dec. 8-11, 1996, pp. 773-776; and Clark et al., xe2x80x9cParallel-Resonator HF Micromechanical Bandpass Filtersxe2x80x9d 1997 International Conference On Solid-State Sensors And Actuatorsxe2x80x9d, pp. 1161-1164.
Due to strict requirements on both long- and short-term stability, the local oscillator (LO) synthesizer is among the most difficult functional blocks to implement in a transceiver, and among the largest power consumers in the total subsystem. In most renditions of local oscillator synthesizers, two oscillators are utilized: a high-frequency (0.4-4 GHz) voltage-controlled oscillator (VCO) referenced to a medium Q (Qxcx9c40) off-chip LC tank; and a lower frequency (xcx9c10 MHz) reference oscillator using a quartz crystal with a loaded Qxcx9c10,000. In the synthesizer, the VCO generates the LO output, and is able to achieve good far-from-carrier phase noise, but must be locked via a phase-locked loop to the reference oscillator to attain adequate thermal stability and sufficiently low close-to-carrier phase noise. The power consumption in such synthesizers is often dominated by the requirement for low phase noise (xcx9c30 mW needed in the VCO for short-range applications), and by the need for phase-locking and frequency division when locking the two oscillators (xcx9c70 mW needed). If oven-control is required for stability against thermal variations, an additional 1-10 W may be required, not to mention a much larger size.
An object of the present invention is to provide a method and apparatus for generating a signal having at least one desired output frequency in a system in response to a tuning voltage and without the need for a phase-locking circuit by utilizing a bank of vibrating micromechanical devices.
In carrying out the above object and other objects of the present invention, a method is provided for generating a signal having at least one desired output frequency in a system without the need for a phase-locking circuit. The method includes providing a bank of micromechanical devices and vibrating one or more of the devices so that each vibrating device generates a signal having an output frequency. The method also includes controllably selecting one or more of the vibrating devices to generate a signal having the at least one desired output frequency in response to the tuning voltage.
Each of the devices may have a Q in excess of 100 and, preferably, more than 5000.
The number of devices may be more than 10, and may even be more than 1000.
The desired output frequency may be more than 300 MHz.
Each of the devices may have an adjustable Q. The method may further include lowering the Q of the at least one vibrating device having the at least one desired output frequency initially during the step of vibrating and then raising the Q during the step of vibrating. The step of lowering may include controllably coupling at least one circuit element to the at least one vibrating device having the at least one desired frequency and the step of raising may include the step of controllably decoupling the at least one circuit element from the at least one vibrating device having the at least one desired frequency.
In further carrying out the above object and other objects of the present invention, a local oscillator synthesizer apparatus for generating a signal having at least one desired output frequency is provided for use in a system in response to a tuning voltage and without the need for a phase-locking circuit. The apparatus includes a parallel bank of micromechanical resonators. Each of the resonators corresponds to a frequency. A controller is provided for selecting at least one desired resonator of the bank of resonators to thereby provide frequency selection.
The apparatus further includes an oscillator sustaining circuit wherein each of the resonators is switchable into or out of the oscillator sustaining circuit by the controller.
The controller may include a decoder for controlling application of an appropriate bias voltage and to the at least one desired resonator.
The resonators may be interlinked and wherein each of the resonators may have an input connected to a common input and also may have an output connected to a common output.
The frequency of each of the resonator devices is not only switchable but also tunable (i.e., via dc-biasing) to obtain a bank of switchable, tunable resonator devices.
Each of the resonators has a predetermined Q and the controller may selectively switch at least one circuit element to the at least one desired resonator to lower the Q of the at least one desired resonator from its predetermined Q and selectively switches the at least one circuit element from the at least one desired resonator to raise the Q of the at least one desired resonator to its predetermined Q.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.